The hunt for the woman, spotted close to where the three-year-old went missing, is the critical lead in the investigation and has been described as “a hugely significant line of inquiry”.

The news comes as Madeleine’s parents gave a moving interview as the 10th anniversary of her disappearance approaches and vowed to do “whatever it takes for as long as it takes” to find her.

Sources revealed that police are poised to question the woman, not currently living in Portugal, about her whereabouts when Madeleine vanished from her bed in the Algarve resort of Praia da Luz on May 3, 2007.

An insider said: “Detectives have scoured Europe looking for this woman who is thought to hold the key to solving the entire case.

“After months of tireless police work they will soon be in a position to move in and finally get some answers after a decade of dead-ends. It is a hugely significant line of inquiry that officers hope could lead to an arrest.”

Last month we revealed that the police had identified a person they wanted to question having been given an extra £85,000 to follow up the crucial lead.

The Scotland Yard investigation, codenamed Operation Grange, has cost £11million since being launched in 2011 and was due to end within weeks as funding ran out.

But the fresh development means funding for the investigation will continue until September.

The Sunday Express is withholding some of the details of the investigation gleaned from sources so as not to compromise the case. Police have looked at more than 600 individuals who were identified as being potentially significant.

The international hunt to find the key woman comes as Kate, 48, and Gerry McCann, 49, of Rothley, Leicestershire, gave an interview to BBC news presenter Fiona Bruce in Loughborough.

Asked about Scotland Yard pursuing a “critical” line of inquiry, Kate said: ‘‘We’ve come a long way and there is progress and there are some very credible lines of inquiry that the police are working on.

“Whilst there’s no evidence to give us any negative news, hope is still there.”

Gerry added: ‘‘They’ve managed to pull so much together and sift through so much information, so now we do seem to be on just several lines of inquiry rather than tens/hundreds.”

No evidence has been produced to show their beloved daughter is dead and suggesting anything opposite to that is “not justice for Madeleine” he said.

A trained anesthetist, Kate quit her job as a GP after Madeleine went missing but revealed she now has a job in medicine.

Fellow doctor Gerry has risen to become one of the country’s foremost heart specialists.

As they approach the unwanted milestone, they are very concerned about their 12-year-old twins Sean and Amelie and how they will cope with vile and abusive internet accusations, fuelled in part by former Portuguese police chief Goncalo Amaral, who suggested Madeleine died in apartment 5a of the Ocean Club.

Amaral won an appeal hearing, allowing him to publish his controversial book on the case called The Truth Of The Lie.

The McCanns said they have been reluctantly forced to take the case all the way to the European Court of Human Rights.

Gerry described the Portuguese appeal court judgment as “terrible”, adding: “We will be appealing.”

Scotland Yard last night said it would not comment on a current investigation.